Mobile trunking radio is well known in the art and is also sometimes known as Special Mobile Radio (SMR) systems. A trunked radio system is a system setup to serve a plurality of users in a licensed radio frequency band to provide radio or radio-telephone services to base or mobile units within a defined geographic market area. Typically SMR systems are used in the public safety industry for such services as police, fire, and medical providers and also in the industrial or land transportation industries, such as, taxicab services, bus services and the like. Such an SMR system typically also includes repeater stations in which the signals from individual users are amplified and repeated at various places within the geographic market to provide a wider geographic accessibility.
In the United States, an SMR service provider typically obtains a license for a band of frequencies within the 800 and 900 MHz frequency range from Federal Communication System (FCC). The SMR system is described as a trunked system which means that several channels are allocated within a given frequency band and the transmitters and receivers use automatic switching so that all users can access any one of the allocated channels which are not currently in use. This form of frequency sharing results in a minimum wait or blockage of a message through the system by an individual user since the user's transmitter shares all available channels.
By way of example, the FCC has allocated 250 channels located in the frequency band of 809 to 816 MHz. Each channel spacing is 25 KHz and the maximum deviation of a transmit frequency is plus or minus 5 KHz. The required frequency stability is 2.5 ppm for transmitters. The FCC allocation also provides for repeater channels which are located 45 MHz above these frequencies. For purposes of the present discussion, repeater frequencies are not required and hence will not be discussed.
Typically the mobile transceivers in an SMR system can be linked to a public switched telephone network by base station or to cellular telephone systems. Access to the public switched network may be from a variety of means including DTMF tone signalling for automatic operation without human intervention. All the SMR channels are voice channels for carrying analog voice signals to in-band tone signalling may be used to interface to a land-based public switched telephone network. Links to cellular telephone systems has been done in systems described as Extended Special Mobile Radio (ESMR) systems.
Special mobile radio systems to which the present invention applies allow the automatic sharing of a plurality of channels by all users of the system in a blocking communication model. The system is blocking since there are typically more users than there are channels and the channels are allocated based on demand. Statistically the number of channels allocated to a user group is based upon a peak demand usage to minimize blockage of the channels due to unavailability of an open channel. Of course the greater number of channels in a special mobile radio system, the fewer times the channels are blocked.
Another feature of the special mobile radio system is the use of a control channel to allow coherent channel allocation to maximize usage of the system. The control channel may be a dedicated frequency, a subcarrier frequency or a sub-audible frequency in selected voice channels in the SMR system. Allocation and usage of the communication channels is therefore negotiated through the control channel in a distributive processing topology in which a central control site and the various user sites negotiate the use of the channels during handshake transmissions prior to channel usage. Various types of communications protocol may be employed to ensure non-interference among the various users of the system. The control channel typically carries control information in a digital format.
Prior art FIG. 1 shows a typical simplified SMR radio system used to communicated with a plurality of cars 101a, 101b, 101c, etc. from a control station 103 via communications antenna 102. In this system, all the mobile radio transceivers 101a, 101b, 101c share a number of common frequency channels in which an individual transceiver may use any one of the unused channels at any given time to communicate with the control station.
There is a need in the art, however, to locate the mobile transceivers in a special mobile radio system. There is a need to identify and locate the mobile transmitters in an SMR system while the mobile transmitters are transmitting. There is also a need in the art to track the movement of the transmitters even if the transmitters change frequency channels over a period of time.
Location of radio transmitters is a technology that has been used for many years and in many applications. For radio location, the use of mobile transmitters and a plurality of fixed base station receivers is generally known in the art where the fixed receivers locate the source of the radio signal by triangulation of the radio signals. Triangulation is a technique where the fixed base station receivers each determine the approximate direction of the radio signal from the transmitter. The approximate direction lines are then drawn on a map where the lines will cross. Since the direction lines are approximate, and typically three fixed base station receivers are used, the intersecting area between the three lines is usually shaped like a triangle since the lines rarely meet at a single point. The transmitter is then located within the triangle.
There is a need in the art for a ground-based vehicle or mobile transmitter location system which is capable of locating SMR transmitters. There is a further need for a ground-based mobile transmitter location system which is immune from disruption of the radio signals in an urban or irregular terrain environment. For these reasons, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a special mobile radio vehicle or personal locator system.